In order to manage radio resources efficiently in a broadband wireless access network, the characteristics of the wireless link are adapted. Link adaptation relies upon receiver channel condition measurements such as carrier to interference-noise ratio (CINR), received signal strength indicator (RSSI), noise and interference levels, instantaneous capacity, a number of retries and a number of packets lost. In particular, an accurate estimate of the carrier to interference-noise ratio (CINR) is required. This estimate allows radio signal strengths to be controlled.
In the Institute of Electrical and Electronic Engineers (IEEE) 802.16 Broadband Wireless Access standards for communication networks (also known as “WiMAX”), the carrier to interference-noise ratio (CINR) is an important air interface condition indicator that is used for resource management of the Physical Layer of the network, and CINR estimates are mandated for network base stations. The WiMAX specification recommends that the CINR be calculated as
                                          CINR            k                    =                                                    ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                                                      s                                          k                      ,                      n                                                                                        2                                                                    ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                                                                            r                                              k                        ,                        n                                                              -                                          s                                              k                        ,                        n                                                                                                              2                                                    ,                            (        1        )            where rk,n is the received sample n within signal k; sk,n represents a detected or pilot sample with channel state weighting; and N is the number of samples used in the estimate. It is also stated in the WiMAX specification that the estimate should be accurate to within +/−2 dB.
This CINR estimation method is suitable for multiple receive antennas that use maximum ratio combining (MRC) or non-SDMA (space division multiple access) systems. In these applications, the CINR for each antenna is calculated separately then added up to form a total signal quality indicator. However, in case of SDMA or MIMO (multiple-input/multiple-output) systems, this estimate is no longer valid, because antenna beam-forming or MIMO equalizer results in some interference cancellation. In these systems, the effective interference seen in the output of a MIMO equalizer or a beam-former is not a summation of interference on each antenna.
It has been noted that the traditionally estimated interference-plus-noise, used for MCS (modulation code scheme) selection in a MIMO and SDMA Scheduler, should be offset by a value, due to interference seen in decoder has been changed after beam-forming. However, no methods have been disclosed for determining the offset value. In order to manage radio resource efficiently, a more accurate CINR estimate is required when an antenna beam-former or a MIMO equalizer is employed in base station or subscriber station.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.